DNA topoisomerases represent a unique class of nuclear enzymes that alter the topological state of DNA by breaking and rejoining the phosphodiester backbone of DNA. Mammalian topoisomerase I is capable of altering the topology of DNA by transiently breaking one DNA strand, while topoisomerase II acts by causing double-strand breaks. Topoisomerase poisoning has recently been recognized as an attractive pharmacological target for the development of novel cancer chemotherapeutic agents. Alkaloids and their derivatives have been investigated as potential antitumor agents, including camptothecin and berberine (Hahn et al., Antibiotics, Vol. 3, Gottlieb et al. (eds.), Springer: New York, pp 577-584 (1975); Shideman, Bull. Natl. Formulary Committee, 18:3 (1950); Bhakkuni et al., The Alkaloids; Vol. 28, Brossi, A. (ed.), Academic Press: New York, pp 95-181 (1986); Suffness et al., The Alkaloids; Vol. XXV, Brossi, A. (ed.); Academic Press: New York, pp 178-197 (1985)).
Extensive studies of camptothecin and its derivatives have established that cellular topoisomerase I is the molecular target for the antitumor alkaloid camptothecin (Hsiang et al., Cancer Res. 48:1722-1726 (1988)). However, some lines of tumor cells have demonstrated resistance to camptothecin.
The demonstration that topoisomerase I is the molecular target for camptothecin has stimulated further identification and development of other topoisomerase I-targeting antitumor compounds (topoisomerase I poisons). Among these are actinomycin D, morpholinodoxorubicin, DNA minor groove binding bis- and tris-benzimidazoles, indolocarbazole derivatives, bulgarein, intoplicine, saintopin, indoloquinolinediones, nitidine derivatives and berberine derivatives. Many of these compounds are also dual poisons of both topoisomerase I and II. Although the number of new topoisomerase I poisons is increasing rapidly, except for camptothecin, topoisomerase I-poisoning has not been demonstrated to be responsible for cell killing for any of the new poisons. Similarly, it is unclear whether the intercalative mode of DNA binding, which is known to be essential for topoisomerase II poisoning, is responsible for poisoning topoisomerase I for the dual poisons.
Many studies have focused their attention on a widely distributed class of alkaloids, whose structures are related to the isoquinoline ring. Two of these compounds, the benzophenanthridine, nitidine, and the related alkaloid fagaronine have a high potency in inducing topoisomerase I-mediated DNA cleavage in vitro (Wang et al., Chem. Res. Toxicol. 6:813-818 (1993)). Several compounds belonging to the family of protoberberines, which are known for their antitumor activities in animals, have also been shown to be topoisomerase I poisons. Berberine represents one of the most intensively studied of the naturally-occurring protoberberine alkaloids and is reported to exhibit weak antitumor activity against P-388 leukemia in mice (Suffness et al., The Alkaloids, Vol. XXV, Brossi, A. (ed.), Academic Press: New York, pp 178-197 (1985)).
Coralyne, an alkaloid analog of protoberberine, has more pronounced antitumor activity relative to berberine, exhibiting significant activity in vivo in mice against L1210 and P388 leukemias (Zee-Cheng et al., J. Med. Chem. 17:347 (1974); Zee-Cheng et al., J. Med. Chem. 19:882 (1976)). While the molecular basis for its antitumor activity has not been identified, it has been speculated that the ability to bind to duplex and triplex DNA may contribute to the observed antileukemic activity (Wilson et al., J. Med. Chem. 19:1261 (1976); Lee et al., Biochemistry 32:5591-5597 (1993)). The antitumor activity of coralyne, coupled with its relatively low toxicity, prompted studies on the synthesis of a number of derivatives (Zee-Cheng et al., J. Med. Chem. 17:347 (1974)), and have suggested that the presence of the methyl substituent at the 8-position and unsaturation at the 5,6-position of coralyne are strongly associated with their antitumor activity against L1210 and P388 leukemias in mice (Messmer et al., J. Pharm. Sci. 61:1858-1859 (1972); Cushman et al., J. Med. Chem. 28:1031-1036 (1985); Stermitz et al., J. Med. Chem. 18:708-713 (1975); Janin et al., J. Med. Chem. 36:3686-3692 (1993)).
However, despite efforts to develop safe and effective therapeutics, the need exists for anti-cancer agents with improved cytotoxicity and which are effective against drug-resistant cancer cells.